A liquid chromatograph is an instrument for analyzing liquids into which a sample to be analyzed is introduced via a stream of solvent from an injector. The flowing stream of solvent, known as the mobile phase, forces the sample through a narrow bore transport tube to a column. The column is a larger diameter tube packed with a bed of small particles known as the stationary phase.
The sample mixture is separated as a result of differential partition between the stationary and mobile phases and as the mobile phase is forced through the stationary phase, a multiple component sample is separated into discrete zones or bands. The bands continue to migrate through the bed and eventually passes out of the column (a process known as elution) and through any one of number of detectors which provide input to a recording device, one example being a strip chart recorder. A deflection of the pen of the recorder indicates the elution of one or more chromatographic bands. Recorder tracing from the elution of a single band is called a peak and the collection of peaks which result from an injected sample comprise the chromatogram. Peaks are usually identified by their retention time or volume.
A phenomenon encountered in liquid chromatography is called "peak spreading" which results from the sample being excessively diluted in the injector apparatus before entering the column. Consequently, the peaks appearing on the analytical chart which are descriptive of the composition leaving the column, are less distinct, i.e., they are "spread" into lower less definable shapes.
It is one of the objectives of this invention to address the problem of "peak spreading".
Numerous prior art mechanisms have been developed with this objective in mind. One device is shown in U.S. Pat. No. 3,916,692 to Abrahams et al. A basic flow pattern is described in the Abrahams et al. patent; a parallel path conduit is employed, one path is a sample loop conduit and the second path is a primary conduit. The conduits are connected in parallel in the loading phase and a liquid sample to be tested is loaded into the sample loop until it is filled. At this time, the sample loop is not in communication with the primary conduit.
The primary conduit includes flow restricting means or bypass restrictor and when a solvent or mobile phase is pumped through the primary conduit, none of the mobile phase (also known as the carrier fluid) flows through the sample loop. The flow of the carrier fluid is blocked by closed valves during the time the sample loop is being filled. Upon the opening of the valves which heretofore blocked the flow of carrier fluid through the sample loop, the high pressure carrier fluid forces the sample from the loop into the column containing the stationary phase. Because of the presence of the bypass restrictor in the primary conduit, essentially most of the flow but not all of the carrier fluid is diverted to the sample loop to force the sample into the column. The Abrahams et al. patent disclosed a mechanism for accomplishing this.
Subsequently, U.S. Pat. No. 4,094,196 to Friswell disclosed a liquid sample injector employing a sliding spool valve. It employs a hollow needle with an opening at one end, movable longitudinally to a position where the opening is immersed in a liquid sample. The sample is drawn into the needle and then the needle is moved to a second position with the opening in the main solvent stream. Valves are actuated which divert the main solvent stream through the needle sweeping the sample out to the system for analysis. Sample injectors made in accordance with the Friswell patent have proven to be commercially successful being somewhat complex to manufacture.
It is, thus, another object of this invention to produce a liquid sample injector which is easy to manufacture and which will substantially reduce peak spreading by reducing to a minimum the dilution of the sample in the high pressure liquid phase stream.